1. Field of the invention
The invention relates to a method for fabricating a solid-state imaging device in which a metallic light shield film is formed to avoid light coming into an area other than a light receiving sensor.
2. Description of Related Art
FIGS. 4A to 4D show steps of a conventional fabricating process of a solid-state imaging device. Conventionally, a solid-state imaging device is obtained by forming a metallic light-shield film 9 on the light receiving sensor 2 and the transfer electrode 7, as shown in FIG. 4A, after forming a light receiving sensor 2 that receives incoming light and saves it as electric charge and a transfer electrode 7, which transfers the saved electric charge on a surface layer of a wafer 1.
In addition, as shown in FIG. 4B, an opening is formed on the light receiving sensor 2 of the metallic light-shield film 9 by etching, and, as shown in FIG. 4C, an interlayer film 13 is formed thereon by a CVD process to embed the light receiving sensor 2 and the transfer electrode 7. Then, the interlayer film 13 is reflowed by heat treatment or the like to have the shape of a lens, and a lens material 14 is filled to embed the reflowed interlayer film 13 so as to form an inner-layer lens.
FIG. 5 is a cross-sectional view of the solid-state imaging device obtained in accordance with the above-described process. In a conventional solid-state imaging device, light converged through an interlayer lens is photoelectrically converted when it enters through the light receiving sensor 2 from an opening in the metallic light-shield film 9. It is, however, impossible to suppress a reflective component which enters into the transfer electrode 7 as a reflection off of the metallic light-shield film 9, resulting in the incoming light being smeared. Such a component will hereinafter be referred to as a “smear component”.
Conventionally, there is a solid-state imaging device in which an anti-reflection film is formed to prevent light reflected off the interlayer film constituting the interlayer lens or the like from entering into the imaging device (see, for example, Japanese Patent Application Publication No. Hei 11-103037, hereinafter referred to as “Patent Document 1”). This solid-state imaging device is made according to a process further including a step of forming the anti-reflection film on the interlayer film and a step of forming an anti-reflection film on the inner-layer lens, in addition to the conventional fabrication process of a solid-state imaging device.
In addition, there is another solid-state imaging device in which the reflectivity of a metal light-shield film is lowered by forming the metal light-shield film composed of a refractory metal silicide or the like on an interlayer dielectric, poly-crystallizing the metal light-shield film with a heat treatment on a wafer on which the metal light-shield film is formed at a high temperature of 800° C. or higher, and oxidizing a surface of the poly-crystallized metal light-shield film (see, for example, Japanese Patent Application Publication No. Hei 8-78651, hereinafter referred to as “Patent Document 2”).
As disclosed in Patent Document 1, in order to obtain a solid-state imaging device in which an anti-reflection film is formed on the interlayer film and on the inner-layer lens, it is necessary that the fabrication process further includes a step of newly forming an anti-reflection film in addition to the conventional fabrication process of a solid-state imaging device. The step of forming the anti-reflection film is carried out in an apparatus different from that used for the steps before and after the formation thereof. Accordingly, it takes time to convey the films into and out from each apparatus, such that the time duration until completing the product fabrication will be long.
In addition, Patent Document 2 discloses that the surface of the metal light-shield film is oxidized at a high temperature of 800° C. or higher. Refractory metal silicide is the only refractory metal which can withstand such a high temperature. Since a light-shield film composed of a metal such as tungsten (W) and molybdenum (Mo) will oxidize too much under heat treatment in an oxidizing atmosphere at a temperature of 800° C. or higher, even though it is a refractory metal, it cannot be used as a light-shield film.